Scarcity of fixed carbon transfer in a model microbial phototroph–heterotroph interaction

Although the green alga Chlamydomonas reinhardtii has long served as a reference organism, few studies have interrogated its role as a primary producer in microbial interactions. Here, we quantitatively investigated C. reinhardtii's capacity to support a heterotrophic microbe using the established coculture system with Mesorhizobium japonicum, a vitamin B12-producing α-proteobacterium. Using stable isotope probing and nanoscale secondary ion mass spectrometry (nanoSIMS), we tracked the flow of photosynthetic fixed carbon and consequent bacterial biomass synthesis under continuous and diurnal light with single-cell resolution. We found that more 13C fixed by the alga was taken up by bacterial cells under continuous light, invalidating the hypothesis that the alga's fermentative degradation of starch reserves during the night would boost M. japonicum heterotrophy. 15NH4 assimilation rates and changes in cell size revealed that M. japonicum cells reduced new biomass synthesis in coculture with the alga but continued to divide-a hallmark of nutrient limitation often referred to as reductive division. Despite this sign of starvation, the bacterium still synthesized vitamin B12 and supported the growth of a B12-dependent C. reinhardtii mutant. Finally, we showed that bacterial proliferation could be supported solely by the algal lysis that occurred in coculture, highlighting the role of necromass in carbon cycling. Collectively, these results reveal the scarcity of fixed carbon in this microbial trophic relationship (particularly under environmentally relevant light regimes), demonstrate B12 exchange even during bacterial starvation, and underscore the importance of quantitative approaches for assessing metabolic coupling in algal-bacterial interactions

Taphonomy of Biosignatures in Microbial Mats on Little Ambergris Cay, Turks and Caicos Islands

Microbial mats are taxonomically and metabolically diverse microbial ecosystems, with a characteristic layering that reflects vertical gradients in light and oxygen availability. Silicified microbial mats in Proterozoic carbonate successions are generally interpreted in terms of the surficial, mat building community. However, information about biodiversity in the once-surface-layer can be lost through decay as the mats accrete. To better understand how information about surface microbial communities is impacted by processes of decay within the mat, we studied microbial mats from Little Ambergris Cay, Turks and Caicos Islands. We used molecular techniques, microscopy and geochemistry to investigate microbial mat taphonomy – how processes of degradation affect biological signatures in sedimentary rocks, including fossils, molecular fossils and isotopic records. The top < 1 cm of these mats host cyanobacteria-rich communities overlying and admixed with diverse bacterial and eukaryotic taxa. Lower layers contain abundant, often empty, sheaths of large filamentous cyanobacteria, preserving their record as key mat-builders. Morphological remains and free lipid biomarkers of several bacterial groups, as well as diatoms, arthropods, and other eukaryotes also persist in lower mat layers, although at lower abundances than in surface layers. Carbon isotope signatures of organic matter were consistent with the majority of the biomass being sourced from CO2-limited cyanobacteria. Porewater sulfide sulfur isotope values were lower than seawater sulfate sulfur isotope values by ∼45–50‰, consistent with microbial sulfate reduction under sulfate-replete conditions. Our findings provide insight into how processes of degradation and decay bias biosignatures in the geological record of microbial mats, especially mats that formed widely during the Proterozoic (2,500–541 million years ago) Eon. Cyanobacteria were the key mat-builders, their robust and cohesive fabric retained at depth. Additionally, eukaryotic remains and eukaryotic biosignatures were preserved at depth, which suggests that microbial mats are not inherently biased against eukaryote preservation, either today or in the past

OA01-06 LB. HIV-1 plasma RNA and risk of HIV-1 transmission

Background: Non-sterilizing HIV-1 vaccines may provide public health benefits if they significantly reduce plasma HIV-1 RNA, thus potentially reducing infectiousness. Quantification of reduction in plasma HIV-1 RNA needed to decrease HIV-1 transmission is useful for design of efficacy trials of candidate HIV-1 vaccines. We modeled the relationship between plasma HIV-1 RNA and HIV-1 transmission using data from a prospective study of African heterosexual HIV-1 serodiscordant couples. Methods: 3408 HIV-1-infected participants with CD4 counts ≥250 cells/mm3 enrolled in the Partners in Prevention HSV/HIV Transmission Study and their partners were followed for ≤24 months. HIV-1 transmission events were assessed for viral genetic linkage within the enrolled partnership by determining HIV-1 env and gag sequences from partners. The relationship between plasma HIV-1 RNA over time and risk of genetically linked HIV-1 transmission was evaluated with a Cox model with a natural cubic spline. Results: 84 post-enrollment linked HIV-1 transmissions were observed. HIV-1 incidence increased rapidly and non-linearly with higher plasma HIV-1: from 0.53 transmissions per 100 person-years for plasma HIV-1 RNA 1,000,000 copies/mL (p<0.0001). Baseline HIV-1 RNA in men was, on average, 0.4 log10 higher than in women; no significant difference in risk of transmission for a given HIV-1 level was observed between men and women (p = 0.17). Given the distribution of plasma HIV-1 RNA in this population of stable cohabiting couples, our modeling predicts that a 0.74 log10 reduction in average plasma HIV-1 RNA in the population would be required for a 50% reduction in HIV-1 transmission risk. Conclusion: This analysis provides a detailed description of the relationship between plasma HIV-1 RNA and risk of heterosexual HIV-1 transmission. These findings suggest targets for reduction in HIV-1 RNA for use in evaluating non-sterilizing HIV-1 vaccine candidates in HIV-1 infected persons to reduce risk of heterosexual HIV-1 transmission

Biogenesis and transmembrane topology of the CHIP28 water channel at the endoplasmic reticulum.

CHIP28 is a 28-kD hydrophobic integral membrane protein that functions as a water channel in erythrocytes and renal tubule epithelial cell membranes. We examined the transmembrane topology of CHIP28 in the ER by engineering a reporter of translocation (derived from bovine prolactin) into nine sequential sites in the CHIP28 coding region. The resulting chimeras were expressed in Xenopus oocytes, and the topology of the reporter with respect to the ER membrane was determined by protease sensitivity. We found that although hydropathy analysis predicted up to seven potential transmembrane regions, CHIP28 spanned the membrane only four times. Two putative transmembrane helices, residues 52-68 and 143-157, reside on the lumenal and cytosolic surfaces of the ER membrane, respectively. Topology derived from these chimeric proteins was supported by cell-free translation of five truncated CHIP28 cDNAs, by N-linked glycosylation at an engineered consensus site in native CHIP28 (residue His69), and by epitope tagging of the CHIP28 amino terminus. Defined protein chimeras were used to identify internal sequences that direct events of CHIP28 topogenesis. A signal sequence located within the first 52 residues initiated nascent chain translocation into the ER lumen. A stop transfer sequence located in the hydrophobic region from residues 90-120 terminated ongoing translocation. A second internal signal sequence, residues 155-186, reinitiated translocation of a COOH-terminal domain (residues 186-210) into the ER lumen. Integration of the nascent chain into the ER membrane occurred after synthesis of 107 residues and required the presence of two membrane-spanning regions. From this data, we propose a structural model for CHIP28 at the ER membrane in which four membrane-spanning alpha-helices form a central aqueous channel through the lipid bilayer and create a pathway for water transport

Active Ooid Growth Driven By Sediment Transport in a High-Energy Shoal, Little Ambergris Cay, Turks and Caicos Islands

Ooids are a common component of carbonate successions of all ages and present significant potential as paleoenvironmental proxies, if the mechanisms that control their formation and growth can be understood quantitatively. There are a number of hypotheses about the controls on ooid growth, each offering different ideas on where and how ooids accrete and what role, if any, sediment transport and abrasion might play. These hypotheses have not been well tested in the field, largely due to the inherent challenges of tracking individual grains over long timescales. This study presents a detailed field test of ooid-growth hypotheses on Little Ambergris Cay in the Turks and Caicos Islands, British Overseas Territories. This field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds. This configuration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time. Ooid size, shape, and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading. Ooid surface textures, petrographic fabrics, stable-isotope compositions (δ^(13)C, δ^(18)O, and δ^(34)S), lipid geochemistry, and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic (e.g., abiotic precipitation, physical abrasion) and biologically influenced processes (e.g., biologically mediated precipitation, fabric destruction through microbial microboring and micritization) to grain size and character. A convergence of evidence shows that active ooid growth occurs along the transport path in a high-energy shoal environment characterized by frequent suspended-load transport: median ooid size increases by more than 100 μm and bulk radiocarbon ages decrease by 360 yr westward along the ∼ 20 km length of the shoal crest. Lipid and 16S rRNA data highlight a spatial disconnect between the environments with the most extensive biofilm colonization and environments with active ooid growth. Stable-isotope compositions are indistinguishable among samples, and are consistent with abiotic precipitation of aragonite from seawater. Westward increases in ooid sphericity and the abundance of well-polished ooids illustrate that ooids experience subequal amounts of growth and abrasion—in favor of net growth—as they are transported along the shoal crest. Overall, these results demonstrate that, in the Ambergris system, the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode. Microbes play a largely destructive, rather than constructive, role in ooid size and fabric